4.2.2 Flow characteristics observed during the Gaudex
The extensive Gaudex surface observations permit to determine the flow character-istics. As presented in Mobbs et al. (2005), Lewis (2006) and Lewis et al. (2007) three characteristic flow features could be identified on the Gaudergrat Ridge based on wind speed and direction comparisons:
- flow separation from the ridge crest with a horizontal axis eddy in the lee and re-attachment near the foot of the slope
- flow separation at the north end of the ridge with a stable vertical axis eddy - flow parallel to the ridge at lower altitudes, on both sides and a pronounced
cross-ridge flow independent of the inflow conditions
These flow features could be qualitatively reproduced using ARPS and a com-mercial fluid dynamic code CFX (Hug et al., submitted), using theoretical soundings for a horizontally homogeneous initial state (i.e. the same conditions at every grid points). However, this method works only for short computing periods only, and CFX was able to reproduce the flow separation only in the case of a stable atmo-sphere. In addition to the prevailing west to north-west winds already observed at the Gaudergrat ridge in winter (Gauer, 2001) during snowdrift events, south-west winds were often present during Gaudex 2003. Airflow differences between winter and summer can be explained by thermal effects (Hug et al., submitted).
4.3 Meteorological situation and flow characteristics on Au-gust 11 and 18, 2003
In order to analyse situations as close as possible to winter, this study concentrates on two strong wind days during the period of observations with winds from south-west to north-south-west. These wind conditions are most frequently encountered during snowdrift events at the Gaudergrat. However the thermal effects, although small cannot be neglected during these two summer days.
The two selected days were fair-weather days. The surface isobaric map of the Met Office (Figure 4.1, upper graphic) shows however on the 11th of August that a week low pressure centre (1013hPa) was crossing Switzerland. On August 18, the pressure distribution at the surface showed a synoptic west wind influence.
Figure 4.1: Surface isobaric maps from the Met Office at 12:00 UTC on August 11 (above) and August 18 (below), 2003
A more regional overview of the flow conditions is given by another permanent weather station: the Weissfluhjoch, located 1 kilometer on the south-east of the Gaudergrat, at the altitude of 2693m. The measurements at Weissflujoch show a
4.3 Meteorological situation and flow characteristics on August 11 and 18, 2003 73
north-west flow on the 11th of August before 6:00 and in the afternoon. The wind blows from north-east between 6:00 and 12:00, and from south after 18:00. On the 18th of August, the flow is a steady southerly wind until 6:00, before turning briefly to north and fluctuating between south and west (Figure 4.2) during the rest of the day due to synoptic forcing. The wind speeds are quite constant and range between 2 and 5m/s on August 11 with a peak after 18:00 at 7m/s. On August 18, wind speed is less steady and varies between 1 and 6 m/s.
The local situation, on the upwind side of the ridge, is described by measure-ments at site 27 (Figure 4.3) where the horizontal windspeed and direction are measured at 5m above the ground. On August 11, wind conditions are under the influence of thermal effects but the situation is still interesting for its northwest-erly afternoon flow. As typically observed during Gaudex (Lewis, 2006) at night a southerly wind is blowing. Then during sun rise, the wind direction turns rapidly to east, then north and changes to south-west until midday before blowing from north-west in the afternoon. The wind speed also shows a clear cycle along the day, with a stronger variability due to thermal effects. The wind speed is 3m/s in average between 7:00 and 17:00, whereas it is only about 1m/s during the night. On August 18, the wind direction is south to south-west before turning to the east between 15:00 UTC and 18:00 UTC. Two accelerations are noticeable at 8:30 UTC and 14:00 UTC, the first one corresponding to the start of an increased synoptic forcing at the Weissfluhjoch the second one corresponding to a change in the wind direction.
Concerning the stratification, no sounding are available on these days. An estimation of the atmosphere stability close to the surface is given by the surface heat fluxes as presented in Figures 4.4 and 4.5. The surface heat fluxes are computed using the ultrasonics temperature measurements at 7.5 meters above the surface from sites 31 (East slope), 32 (Crest) and 33 (West slope). The influence of the sun appears clearly on August 11 (Figure 4.4). As soon as the sun light the ground surface, the air close to the ground surface is warmed up and the atmosphere near the ground becomes unstable. Indeed This phenomenon starts earlier on the East slope.
On August 18, as the wind is stronger, the effect of the sun is not so pronounced due to stronger wind. However during the day, the atmosphere become unstable. The surface heat fluxes for both days show that the influence of thermal flow is difficult to avoid during fair weather summer days.
11−Aug−2003 00:000 06:00 12:00 18:00 12−Aug−2003 00:00 90
180 270 360
Measurements at the Weissfluhjoch weather station − 30min average
Wind direction (degrees)
11−Aug−2003 00:000 06:00 12:00 18:00 12−Aug−2003 00:00
2 4 6 8
Horizontal wind speed (m/s)
18−Aug−2003 00:000 06:00 12:00 18:00 19−Aug−2003 00:00
90 180 270 360
Measurements at the Weissfluhjoch weather station − 30min average
Wind direction (degrees)
18−Aug−2003 00:00:000 06:00 12:00 18:00 19−Aug−2003 00:00
1 2 3 4 5 6 7
Horizontal wind speed (m/s)
Figure 4.2: Wind direction and speed measurements at the Weissfluhjoch on August 11 (above) and August 18 (below), 2003 (data are averaged over 30 minutes, time is UTC)
4.3 Meteorological situation and flow characteristics on August 11 and 18, 2003 75
11−Aug−2003 00:00 06:00 12:00 18:00 12−Aug−2003 00:00
0 90 180 270
Measurements at site 27 (West) at 5m above ground − 30min average data
Wind direction (degrees)
11−Aug−2003 00:000 06:00 12:00 18:00 12−Aug−2003 00:00
1 2 3 4
Horizontal wind speed (m/s)
18−Aug−2003 00:000 06:00 12:00 18:00 19−Aug−2003 00:00
90 180 270 360
Measurements at site 27 (wiei) at 5m above ground − 30min average data
Wind direction (degrees)
18−Aug−2003 00:000 06:00 12:00 18:00 19−Aug−2003 00:00
2 4 6 8 10
Horizontal wind speed (m/s)
Figure 4.3: Wind direction and speed measurements at site 27, at 5m above the ground, on August 11 (above) and August 18 (below), 2003 (data are averaged over 30 minutes, time is UTC)
11Aug 00:00−0.2 06:00 12:00 18:00 12Aug 00:00
−0.15
−0.1
−0.05 0 0.05 0.1 0.15
wT (m.s−1.K)
31 East slope 32 ridge 33 West slope
Figure 4.4: Surface heat fluxes at sites 31, 32, 33, on August 11 (Time is UTC)
18Aug 00:00−0.2 06:00 12:00 18:00 19Aug 00:00
−0.15
−0.1
−0.05 0 0.05 0.1
wT (m.s−1.K)
31 East slope 32 ridge 33 West slope
Figure 4.5: Surface heat fluxes at sites 31, 32, 33, on August 18 (Time is UTC)